Raman spectroscopy is a valuable analytical tool that employs characteristic vibrational patterns for the identification of molecules, but its direct implementation as a chemical sensing technique is limited due to the low scattering cross section of most Raman active molecules, which leads to low sensitivity. This limitation has been overcome by the discovery of surface enhanced Raman spectroscopy (SERS), which increases the intensity of Raman signals by leveraging the inherent and unique properties of plasmonic nanoparticles (NPs) hence leading to improved detection limits. The interaction of electromagnetic radiation with the oscillating cloud of conduction electrons of plasmonic nanoparticles results in electromagnetic energy confinement around the nanoparticles. By placing a Raman active molecule on or in close proximity to the nanoparticles, and hence within the confined electromagnetic field, it is possible to obtain significant Raman signal amplification. This phenomenon is known as SERS.
The design and fabrication of plasmonic nanoparticles to sustain high electromagnetic field enhancements for SERS have become a field of interest mainly in the analytical research community due to the high demand of ultrasensitive substrates for the detection of environmental pollutants, toxic industrial waste, and chemical warfare, to name a few. In SERS-based experiments, when a Raman active molecule (SERS reporter) is attached to a plasmonic NP, its Raman signal is boosted on average by about 5-6 orders of magnitude, with peaks of 8-10 orders of magnitude in non ensemble-averaged systems. It has been established that SERS signals can also be intensified via assembly of the plasmonic NPs into dimers and small clusters, where the local electromagnetic field enhancement increases by up to a value of 108-1010 at the junction between NPs, which is also known as the “SERS hot spot.” In addition, high confinement of the electromagnetic energy has also been reported to be present at the sharp edges or tips of anisotropic nanoparticles such as nanorods, nanocubes, and nanoprisms, which have also been widely studied and reported in literature as superior SERS substrates.
Recently, star-shaped gold nanoparticles (spherical core structures with protruding sharp tips) have emerged as excellent SERS substrates, where extraordinary field confinement and enhancement can be observed at the acute tips, that can thus act as excellent “hot spots”. (Khoury, C. G. et al. J. Phys. Chem. C 2008, 112, 18849; incorporated herein by reference in its entirety). The optical properties of nanostars have also been found to be strongly dependent on the morphology of the protruding tips. (Alvarez-Puebla, R., Liz-Marzan, L. M.; Garcia, d. A. J. Phys. Chem. Lett. 2010, 1, 2428; incorporated herein by reference in its entirety). This excellent structure-optical property relationship in gold nanostars has been exploited to design substrates for chemical- as well as bio-detection.
Top-down and bottom-up techniques have been equally utilized for the fabrication of SERS substrates. Many elegant NP substrates have been presented with reproducible large-scale periodic arrays by using, for example, electron beam lithography and focused-ion beam lithography. However, the implementation of top-down methods is limited by the high fabrication cost and the constraints in the inter-nanoparticle space tunability necessary to achieve maximum field enhancements. These limitations can be circumvented by bottom-up procedures that are much cheaper and offer flexibility in controlling the inter-nanoparticle spacing, particularly for NP-assembly-guided SERS substrates, easily reaching inter-nanoparticle separations as low as 1 nm. Bottom-up approaches also permit the use of anisotropic NPs, like gold nanostars and nanorods, which exhibit excellent plasmonic properties with size tunability.
Similar to the top-down methods, SERS platforms prepared from the bottom-up can and should be well characterized for quantitative data acquisition, and designed with high reliability and reproducibility by careful and controlled fabrication practices. Successful implementations of these advantageous features are evident in the variety of emerging bottom-up designed SERS substrates with superior SERS enhancement.
Most of the nanoparticle fabrication methods for SERS-based chemical sensing applications have followed the “sandwich architecture”, where a narrow gap between nanoparticles or nanoparticles and a plasmonic film is created, to take advantage of the quantum confinement effect. For example, in fabrication of a bottom-up approach, dithiolated analyte molecules are sandwiched between a gold film and gold nanostars, and enable zeptomolar sensitivity in the detection of 1-naphthalenethiol. However, in practice, this approach limits its use only to dithiolated analytes. A modified version of this approach was reported for the detection of non-functionalized analytes with limits of detection up to 10−5 M. It was reported that the SERS spectrum for such analytes could only be acquired when they were positioned exactly at the junction between the nanostar tips and the gold thin film. This criterion however would be hard to implement in ultrasensitive detection regimes, thus limiting their widespread use. Not only star-shaped nanoparticles but also other anisotropic nanoparticles, fabricated to give rise to quantum confinement effects, have been used in the literature for SERS-based chemical detection.
Even though these approaches have been able to address the detection of different types of analytes, a solution that overcomes the above-described inadequacies and shortcomings is still needed in the engineering of SERS substrates, particularly for ultrasensitive detection of non-functionalized analytes.